The IC packaging is for protecting dies and providing an interface for the communications between the dies and the external circuitry. As electronic devices are continuously driven for smaller dimension and lighter weight, IC packaging is also required to be even more compact and to have higher layout density. To meet these requirements, numerous new technologies of IC packaging are proposed.
Conventional substrates for IC packaging (hereinafter, packaging substrates) are fabricated by directly forming a wiring layer on a CCL (copper coated laminate) base which is made by pressing a copper foil onto a resin core layer. Due to the constraints of the vias (i.e., through holes) and the thickness of the copper foil, CCL-based packaging substrates cannot offer the thickness and layout density required by recent IC packaging applications.
Currently, a number of methods such as Build Up and Semi-Additive Process (SAP) have been utilized to increase the layout density by forming multiple wiring layers on both sides of a core layer of the packaging substrate. However, vias are still required in providing electrical connection between the wiring layers (i.e., interlayer connection) on the core layer's two sides and, therefore, signal interference is inevitable. Also, when only one side of the packaging substrate requires high layout density, using Build Up method adds unnecessary cost and substrate thickness by having multiple wiring layers on the other side of the core layer as well.
Therefore, to increase the layout density and the quality of the electrical signal for packaging substrates so as to meet market demands, the so-called coreless substrate is proposed, which removes the thick core layer and uses blind vias for interlayer connection. In addition, coreless substrate is actually a flexible printed circuit board (PCB) and it can be manufactured using conventional equipments for regular PCB. Therefore, coreless substrate allows manufacturers to make flexible PCBs without additional investment in purchasing equipments specifically for flexible PCBs.